Lack of humidity in household ambiant air can cause several minor physical disabilities which can be very unpleasant to the house inhabitants, such as allergies and nose bleeding. To counter this lack of humidity, it is known to install either a portable humidifier in one or more rooms, or to combine the house heating system with a main humidifier if the heating system uses air conveying ducts.
The problem with the portable humidifier is that its efficiency/price ratio can be very poor. Indeed, the water poured into the humidifier must be heated until it evaporates, and a water heater must thus be included within the humidifier, this to humidify only one room.
To combine a humidifier to the main heating system can indeed become a very costly solution, for the size of this system can be very important.